Adrainage basin is
the streams that flow directly into an ocean
land covered by floodwaters
the land area that contributes water to a stream
the channel of a stream

The land area that contributes water to a stream

Explanation:

A drainage basin represents the land area which is contributing water to a stream. The water contribution can be directly toward a single stream, or toward several streams that flow into a larger, central stream.

Basically, the drainage basin has a stream running through it, in its lowest part. The land is picking up on elevation aside form the stream, and the ridges of the hills or mountains are making up its boundaries.

Some drainage basins are small, and they can occupy only few square km, while some are enormous and can occupy over million square km. As example we can take a small stream like Timjanichka River that has drainage basin of 8 square km, and on the other hand we can take the drainage basin of the Amazon River which is larger that the majority of the countries in the world.